It is often desirable and sometimes important to minimize the intrusion of water through a louver. In such instances, so-called drainable blade louvers should be used. The principal characteristic of drainable blade louvers is the provision at the front, lower edge of each blade of an upwardly extending flange or a trough which catches water that impinges on the blade and prevents it from flowing off the lower front edge of the blade down the front of the louver. The trough opens at one or both ends of the blade into a vertical drainage channel in the vertical mullions or side frames of the louver. Such troughs in the blades of drainable blade louvers have been somewhat successful in limiting water penetration through the louver in that water that flows or drips down from blade to blade in conventional louvers and that is susceptible of becoming entrained in the airflow is eliminated.
Another provision in the design of at least one known form of blade for a drainable blade louver is a vertical offset or step at approximately the mid point of the blade profile which is intended to present a vertical surface or dam for catching water drops entrained in the airflow. Again, such a step provides some reduction in water penetration through the louver.
It must, of course, be recognized that no open louver can be constructed in such a way as to entirely prevent intrusion of water under severe weather conditions. On the other hand, any significant reduction in water entrainment can be of practical importance.